1. Field of the Invention
This disclosure relates to sound insulation multiple layer panels comprising a multilayer interlayer. More specifically, the present invention discloses multiple layer glass laminates having an interlayer thickness factor greater than about 0.80 and comprising a first glass sheet, a second glass sheet, and a multiple layer acoustic interlayer. The multiple layer glass laminates have improved sound insulation at the coincident frequency region of the laminates.
2. Description of Related Art
Poly(vinyl butyral) (PVB) is often used in the manufacture of polymer sheets that can be used as interlayers in multiple layer panels formed by sandwiching the interlayer between two sheets of glass or other rigid substrate. Such laminated glass or glass panel has long served for safety purposes and is often used as a transparent barrier in architectural and automotive applications. One of its primary functions is to absorb energy resulting from impact or a blow without allowing penetration of the object through the glass and to keep the glass bonded even when the applied force is sufficient to break the glass. This prevents dispersion of sharp glass shards, which minimizes injury and damage to people or objects within an enclosed area. Less known is the advantage of laminated glass for noise attenuation. Over the past decades, architectural use of laminated glass in buildings near airports and railways has served to reduce the noise levels inside the buildings, making it more comfortable for the occupants. Likewise this technology is now being used in buildings where street and highway traffic noise is a problem. Recently, advances in interlayer technology have made improved laminated glass that provides noise and vibration improvements for automotive glass.
Traditionally, glass panels used in automotive applications employ two glass sheets each having a thickness between 2.0 and 2.3 millimeters (mm). Most often, these sheets have approximately the same thickness. This type of configuration facilitates both strength and rigidity in the final panel, which, in turn, contributes to the overall mechanical strength and rigidity of the vehicle body. Some estimates attribute up to 30 percent of the overall rigidity of a vehicle to its glass. Thus, the design and rigidity of the multiple layer glass panels used for constructing vehicle glazings such as, for example, the windshield, sun or moon roof, and side and rear windows, are critical not only for the performance of those panels, but also for the overall performance of the vehicle itself.
Recent trends toward more fuel efficient vehicles have brought about demand for lighter weight vehicles. One way of reducing vehicle weight has been to reduce the amount of glass by using thinner glass sheets. For example, for a windshield having a 2.1 mm/2.1 mm glass configuration and a surface area of 1.4 m2, reducing the thickness of one of the panels by about 0.5 mm can result in a weight reduction of over 10 percent, all other things being equal.
One approach to thinner multiple layer panels has been to use an “asymmetric” glass configuration, wherein one of the panels is thinner than the other. Thinner glass panels with symmetric configurations have also been used. However, the asymmetric configurations are more often employed and involve using an “outboard” glass panel (i.e., the glass panel facing outside of the vehicle cabin) with a traditional 2.0 mm to 2.3 mm thickness and a thinner “inboard” glass panel (i.e., the glass panel facing the interior of the cabin). The thicker outboard glass is to ensure adequate strength and impact resistance against rocks, gravel, sand, and other road debris to which the outboard panel would be subjected during use. Typically, however, these asymmetric panels have a combined glass thickness of at least 3.7 mm in order to maintain properties such as deflection stiffness, glass bending strength, glass edge strength, glass impact strength, roof strength, and torsional rigidity within acceptable ranges.
Further, because asymmetric configurations are typically formed by utilizing a thinner inboard glass sheet, the sound insulation properties of these panels are often poorer than similar panels utilizing thicker glass. Therefore, in order to minimize road noise and other disturbances within the cabin, interlayers used to form asymmetric multiple layer panels are generally interlayers having acoustic or sound dampening or sound insulating properties (i.e., acoustic interlayers). Conventional, non-acoustic interlayers do not provide sufficient sound insulation for most applications requiring good sound insulation.
The noise transmission through the glazing is a major contributor to the consumer's perception of vehicle interior noise level. The windshield and side windows are of particular importance for the interior noise level and are one of the acoustic limitations to designing quieter car interiors. Acoustic energy can be transmitted rather easily through windshields and side windows compared to other areas of the passenger compartment boundaries. At high operating speeds, aerodynamic pressure fluctuations resulting from exterior airflow in the vicinity of the windshield and side windows are very strong, causing the glass surface to radiate noise to the vehicle interior. Airflows impinging on panel edges and bends can generate acoustic noise with subsequent airborne transmission to the vehicle interior. The transmission of airborne noise generated by adjacent moving vehicles and structure-borne noise due to structural vibration of the car body also contribute to noise transmission through windshields and side windows.
The sound insulation property of a glass panel can be characterized by Sound Transmission Loss (STL). Glass reacts best to excitation frequencies that matches its natural frequencies. Because of low internal damping, glass resonates readily at low frequencies, which are determined by stiffness, mass and dimensions of a glass panel, and increases sound transmission. Above the low frequency resonant region to below the coincident frequency region (typically 300 Hertz (Hz) to less than 2500 Hz for a windshield or side glass), the mass of glass dominates the sound transmission and the glass follows the mass law of acoustics. In the mass law controlled region, the sound transmission loss increases about 6 decibels (dB) with increasing frequency by one octave band, and increases about 6 dB by doubling the glass thickness or surface density of the glass. Traditionally, the surface density of a glass panel is increased by increasing the thickness of one or both glass sheets.
It is well known that sound transmission through glass exhibits coincident effect. Glass has a specific critical or coincident frequency at which the speed of an incident acoustical wave in air matches that of the glass bending wave. At the coincident frequency, the acoustic wave is especially effective at causing glass to vibrate, and the vibrating glass is an effective sound radiator at or near the coincident frequency and at frequencies above or below the coincident frequency or in the coincident frequency region. As a result, glass exhibits a dip or decrease in sound transmission loss, referred to as the coincidence dip or coincident effect, and the glass becomes transparent to sound.
The coincident frequency can be represented by equation (1):fc=c2/2τ×[ρs/B]1/2  (1)where c is the sound speed in air, ρs is the surface density of the glass panel, and B is the bending stiffness of the glass panel. In general, the coincident frequency increases with decreasing thickness of the glass panel. For automotive glazings, the coincident frequency is typically in the range of 3150 to 6300 Hz, which is well within the wind noise frequency region of 2000 to 8000 Hz. For laminated architectural building glass (such as windows), the coincident frequency is generally less than about 3150 Hz.
The coincident effect not only results in a dip or decrease in sound transmission loss at the coincident frequency, but also reduces sound transmission loss in the coincident frequency region. Glass panels exhibiting severe coincident effect (low STL) at the coincident frequency will transmit sound more dominantly at that frequency, resulting in an enclosed area (such as the interior of a car or a room in a building) with high sound intensity at or near the coincident frequency.
As previously stated, increasing sound transmission loss of a laminated glass in the mass controlled frequency region is traditionally achieved by increasing the surface density of the laminate, such as by increasing the combined thickness of glass. However, this approach results in little to no change, or even a negative change at the coincident frequency of a laminated glass panel that has either a high or low symmetry of glass configuration (further described below). In addition, the sound transmission loss in the frequency range of 3000 to 5000 Hz is adversely affected.
Thus, a need exists for an alternative solution to increase sound transmission loss in the mass controlled frequency region that also improves sound transmission loss in coincident frequency region.